I believe that "heliotropism" is the *process* of *following*
the sun, while the flower itself is named "heliotrope". The
word can also be a color name taken from the flower, I think
a sort of light purple. So for instance, the phrase "heliotrope
gamahaean union" (which turns up in Neil Gaiman's _Books of Magic_
miniseries) refers to a gamahaean union that is heliotrope in color.